1. Field of Invention
The invention relates to an electromagnetic (EM) wave polarizing technique, more particularly, to an asymmetric two-dimensional photonic crystal unit cell array structure and method for generating a non-in-plane polarized EM wave.
2. Description of Related Art
Generally, most of electromagnetic (EM) waves in nature exist in a non-polarized state. Herein, light refers to an EM wave at around a frequency within a specific range. Since the electric field direction of a non-polarized EM wave is not unique in any direction while propagating in space, a non-polarized EM wave is not suitable for (or limited to) certain applications. For example, laser systems, display devices, or optical telecommunications systems and the like all require polarized light, and therefore, a polarizer (a device) to polarize an EM wave will be indispensable for many applications. There are a variety of ways to design a polarizer, and a grating-type or polymer-type polarizer is most commonly in use. However, with the advent of semiconductor techniques, there are other new ways to design a polarizer or to polarize an EM wave.
Nowadays, semiconductor opto-electronic devices, such as light emitting diode (LEDs) and vertical cavity surface-emitting lasers (VCSELs), are widely used as light sources. The brief description below is made with respect to LEDs and VCSELs. FIG. 1 is a schematic cross sectional view of a conventional LED structure. In FIG. 1, a conventional LED includes a transparent substrate 100, a bottom clad layer 102, an active layer 104, and a top clad layer 106, which are formed sequentially on the electrode layer 100. In addition, an electrode layer 108 is formed on the top clad layer 106, and supporting layer 110 is formed below the electrode layer 100. FIG. 1 shows the basic structure of a semiconductor LED, and the detailed structure should be understandable by those ordinary skilled in the art, and will not be described any more.
FIG. 2 shows light emitting mechanism of a LED. When an operation voltage is applied to the top and bottom electrode layers 108, 100, holes 114 and electrons 116 will be driven by an electric field to move toward the active layer 104. The potential distribution of the holes 114 is shown as a potential line 111, and that of the electrons 116 is shown as a potential line 112. When the holes 114 and the electrons 116 recombine and annihilate in the active layer 104, light 118 thus emits according to the energy thereof. This emitted light is generally non-polarized or partially polarized.
FIG. 1 is a simplified schematic view of a conventional LED or a surface-emitting laser, which is used as a light source, but without any polarizing effect. In practical applications, in order to obtain a polarized light, a polarizer is required to convert a non-polarized light into a polarized one. With the advent of semiconductor fabrication techniques and electromagnetic wave theory, it's possible that the structure used to polarize light can be incorporated and integrated within a light emitting device such as LED directly. However, the structure used to polarize light can be achieved only after good design, and many manufacturers now continue to develop more efficient, lower cost, and easily-fabricated EM wave polarizing structure.
Conventionally, a polarized EM wave is produced by polarizing a non-polarized (or a partially polarized) EM wave (or light) through a polarizer, or by the means of combining a polarizer and a light emitting device (LED). The disadvantages of these methods are stated as follows. 1. The conventional polarizer reduces light intensity by more than a half while light propagates through it; 2. As for LED combined with a polarizer, the medium interfaces will cause multiple Fresnel reflection loss; 3. The design and fabrication process of a conventional polarizer are excessively complex and expensive.